Heat dissipation is a major factor in the design of semiconductor devices such as analog and power transistors and especially in high performance digital switching circuits formed at high integration density. It has become the practice to incorporate attachment of a heat sink or other heat removal structure (e.g. a liquid-cooled cold plate) into the design and manufacturing of integrated circuit packages since heat removal is critical to both performance and reliability of the integrated circuits.
For attachment of heat sinks to integrated circuit packages, it has been the practice to use an adhesive which has a relatively good thermal conductivity. However, the thermal conductivity of such materials is still very low compared to metals. For example, the thermal conductivity of a thermally conductive adhesive in current use is only approximately 1.73 W/m degree C.; whereas copper has a thermal conductivity of 395 W/m degree C.
It has been the practice to bond heat sinks to integrated circuit packages with a reworkable thermoplastic adhesive which is initially in the form of a sheet of a thickness designed to provide the proper volume and thickness of the bond. A dispensable formulation like paste or grease is an alternative to an adhesive preform.
A Thermal Interface Material (TIM) is most effective or can give lower thermal resistance if it has higher thermal conductivity and/or can be utilized at narrower bond-line thickness. TIMs generally have two major components, i.e., fillers, such as metal and/or non-metal particles and/or fibers, and a vehicle such as oil or short chain polymers (oligomers). The vehicle can be either silicone or a non-silicone based compound depending on the source, properties desired, application and cost of the paste. There could be additional components such as a dispersant, curing agent, antioxidants, etc.
Use of silicone oil based thermal interface materials (TIM) in microelectronics packaging could pose challenges with re-workability of parts, e.g., chips, substrates, lid and other hardware due to cross contamination by silicone oil from the TIM.
It is known that the thermal conductivity of a paste TIM can be increased by increasing the filler content in the paste formulation, which is the higher conducting ingredient of the two major components. But, as the filler content in the formulation is increased, the paste becomes more and more stiff or rigid and it becomes difficult to compress the paste to smaller bond-line thickness. Often, many TIMs incorporate a fugitive solvent in the mix, to achieve lower compressive force while trying to reach narrower bond-line thickness. It is often found that the fugitive solvent is a relatively volatile liquid that is difficult to contain for longer periods of time, and can reduce the shelf-life of the TIM.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.